Modern computing and display technologies have facilitated the development of systems for so-called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner where they seem to be, or may be perceived as, real. A virtual reality (VR) scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input, whereas an augmented reality (AR) scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the end user.
For example, referring to FIG. 1, an augmented reality scene 4 is depicted wherein a user of an AR technology sees a real-world park-like setting 6 featuring people, trees, buildings in the background, and a concrete platform 8. In addition to these items, the end user of the AR technology also perceives that he “sees” a robot statue 10 standing upon the real-world platform 8, and a cartoon-like avatar character 12 flying by which seems to be a personification of a bumble bee, even though these elements 10, 12 do not exist in the real world. As it turns out, the human visual perception system is very complex, and producing a VR or AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements is challenging.
VR and AR systems typically employ head-worn displays (or helmet-mounted displays, or smart glasses) that are at least loosely coupled to a user's head, and thus move when the end user's head moves. If the end user's head motions are detected by the display subsystem, the data being displayed can be updated to take the change in head pose (i.e., the orientation and/or location of user's head) into account.
As an example, if a user wearing a head-worn display views a virtual representation of a three-dimensional (3D) object on the display and walks around the area where the 3D object appears, that 3D object can be re-rendered for each viewpoint, giving the end user the perception that he or she is walking around an object that occupies real space. If the head-worn display is used to present multiple objects within a virtual space (for instance, a rich virtual world), measurements of head pose can be used to re-render the scene to match the end user's dynamically changing head location and orientation and provide an increased sense of immersion in the virtual space.
Head-worn displays that enable AR (i.e., the concurrent viewing of real and virtual elements) can have several different types of configurations. In one such configuration, often referred to as a “video see-through” display, a camera captures elements of a real scene, a computing system superimposes virtual elements onto the captured real scene, and a non-transparent display presents the composite image to the eyes. Another configuration is often referred to as an “optical see-through” display, in which the end user can see through transparent (or semi-transparent) elements in the display subsystem to view directly the light from real objects in the environment. The transparent element, often referred to as a “combiner,” superimposes light from the display over the end user's view of the real world.
VR and AR systems typically employ a display subsystem having a projection subsystem and a display surface positioned in front of the end user's field of view and on which the projection subsystem sequentially projects image frames. In true three-dimensional systems, the depth of the display surface can be controlled at frame rates or sub-frame rates. The projection subsystem may include one or more optical fibers into which light from one or more light sources emit light of different colors in defined patterns, and a scanning device that scans the optical fiber(s) in a predetermined pattern to create the image frames that sequentially displayed to the end user.
In a typical head-worn VR/AR system, it is desirable to design the display subsystem to be a light-weight as possible to maximize comfort to the user. To this end, various components of the VR/AR system may be physically contained in a distributed system that includes the display subsystem itself, and a control subsystem locatable remotely from the user's head. For example, the control subsystem may be contained in a belt-pack medallion that can be affixed to the waist of the user. Because it is desirable to locate the light source(s) remotely from the head of the user (e.g., in the belt-pack medallion) due to weight, heat, and form factor considerations, the light source(s) must be located with the control subsystem away from the display.
As such, the optical fiber(s) must be routed from the remote light source(s) to the portion of the display subsystem located on the head of the user. For example, referring to FIG. 2, a single-mode optical fiber 20 is used for performing both transmission and scanning functions by propagating the light from the remote light source(s) 22 (transmission function) to the scanning device 24, where it is manipulated thereby to scan light in a predetermined scanning pattern (scanning function).
To prevent color distortion of the image displayed to the user (after the polarized laser light is propagated through the diffractive optics of the display, which is highly polarization sensitive), the polarization of the light injected into the optical fiber(s) from the light source(s) must be maintained through the entirety of the optical fiber(s). In an ordinary optical fiber, two polarization modes (e.g., vertical and horizontal polarization) have the same nominal phase velocity due to the circular symmetry of the fiber. However, tiny amounts of random birefringence in such a fiber, or bending in the fiber, will cause a tiny amount of crosstalk from the vertical to the horizontal polarization mode. And since even a short portion of fiber, over which a tiny coupling coefficient may apply, is many thousands of wavelengths long, even that small coupling between the two polarization modes, applied coherently, can lead to a large power transfer to the horizontal mode, completely changing the wave's net state of polarization. Since that coupling coefficient was unintended and a result of arbitrary stress or bending applied to the fiber, the output state of polarization will itself be random, and will vary as those stresses or bends vary.
Thus, due to the tortuous path that the optical fiber(s) must be routed between the remote light source(s) and the head-mounted display subsystem (which may be quite long when it spans the human neck and torso), the optical fiber(s) may be bent differently (due to user bodily motion, etc.) and thus strained, thereby drastically changing the polarization of the light traveling through the optical fiber(s).
It is known to use a polarization-maintaining optical fiber (PM fiber), which is a single-mode optical fiber in which linearly polarized light, if properly launched into the fiber, maintains a linear polarization during propagation, exiting the fiber in a specific linear polarization state. PM fibers maintain linear polarization during propagation by intentionally introducing a systematic linear birefringence in the fiber, so that there are two well-defined polarization modes that propagate along the fiber with very distinct phase velocities. Several different PM fiber designs can be used to create birefringence in a fiber. For example, the fiber may be geometrically asymmetric or have a refractive index profile that is asymmetric, such as the design using an elliptical cladding, a design using rods of another material within the cladding, or a design using structured-core fibers (e.g., photonic bandgap fibers) to permanently induce stress in the fiber.
Although projection subsystems can be designed with PM fibers in mind, pre-existing scanning devices are designed to operate with non-PM fibers, which exhibit different mechanical properties than do PM fibers. In this case, the scanning device will not operate in the same manner when the non-PM fiber(s) are substituted with the polarization maintaining optical fiber(s). In particular, a PM fiber with highly asymmetrical bending stiffness will have very different dynamics than a non-PM fiber with symmetrical bending stiffness. Consequently, the PM fiber would not achieve a resonant spiral scan, which is the default scanning mode for the display device. Furthermore, the distal ends of the optical fiber(s) are typically tapered to increase operating frequency and thus better resolution, thereby resulting in better performance. However, in the case where the PM fibers utilize stress inducing elements, these additional elements will affect the scanning field of the scanning device. As such, a scanning device would have to be redesigned to accommodate the alternative use of PM fibers, which would result in additional cost to the VR/AR system.
There, thus, is a need for a low cost solution that maintains linear polarization in an optical fiber connected between a remote light source and a head-mounted display subsystem without having to redesign a scanning device in the display subsystem.